(a) Field of the Invention
The present invention relates to an active-matrix LCD (liquid crystal display) device and, more particularly, to an active-matrix LCD device including a color filter substrate having a polished surface. The present invention also relates to a method for manufacturing such a color filter substrate.
(b) Description of the Related Art
Active-matrix LCD devices are increasingly used for display of color images. FIG. 7 shows a conventional LCD device in a sectional view. The active-matrix color LCD device includes a TFT substrate 30 on which thin film transistors (or active elements, not shown in the figure) are formed, and a color filter substrate (or counter substrate) 10A mounting thereon a color filter. The TFT substrate 30 and the color filter substrate 10 oppose each other with a liquid crystal (LC) layer 40 being sandwiched therebetween. There are three (R, G and B) coloring layers 23 provided on the color filter substrate 10A to enable the color image display.
The color filer substrate 10A includes a glass substrate 21, a black matrix (BM) 22 formed thereon and having a substantially lattice pattern, coloring layers 23 each formed on the glass substrate 21 in the space of the lattice of the black matrix 22 and on the edge portion of the stripe of the lattice of the black matrix 22, and a transparent electrode 24 formed on the coloring layers 23 and the other portions of the lattice of the black matrix 22. Each coloring layer 23 has a thickness of around 1 to 3 μm, for example.
The black matrix 22 has spaces within the lattice pattern, prevents leakage of light, shields an alignment defect of the LC layer, and defines the light transmission areas or pixel image areas. The black matrix 22 is made of a chrome film, or a resin film into which carbon etc. is dispersed. In a recent trend, the resin film is increasingly used for the black matrix 22 due to the rising cost of the chrome film.
The surface of each coloring layer 23 has a step difference between a first area, or protruding area, wherein the edge of the stripe of the black matrix 22 and the edge of the coloring layer 23 overlap each other, and a second area, or substantially flat area, wherein the coloring layer 23 is formed directly on the glass filter 21 in the space of the black matrix 22.
The step difference between the first area and the second area causes a problem known as disclination. If the black matrix 22 is made of a chrome film, the disclination problem scarcely arises due to the small thickness, 0.1 to 0.2 μm, of the chrome film which is well smaller than the thickness of the coloring layer 23. On the other hand, if the black matrix 22 is made of a resin film, the disclination problem arises due to the larger thickness of the resin film, which amounts around 1 to 2 μm to achieve a sufficient light shielding property or light absorbing property.
To solve the problem as described above in connection with the resin film of the black matrix, an overcoat film made of acrylic resin or epoxy resin may be provided between the coloring layers 23 and the transparent electrode 24 for improving the surface flatness of the overall color filter substrate 10A. This requires, however, an additional fabrication step to thereby increase the cost for the LCD device. Another technique for improving the surface flatness of the color filter substrate 10A is such that the surfaces of the coloring layers 23 are polished, after forming the coloring layers 23 by using an electrodeposition technique. This technique is described in JP-A-9-230124, for example.
In the conventional technique, the allowable step difference for the surface of the color filter substrate 10A is generally below 0.5 μm, as described in the above patent publication. More specifically, it is generally considered that a step difference below 0.5 μm in the coloring layers is sufficient to suppress the disclination of the LCD device caused by the surface of the color filter substrate.
However, the present inventors found from a variety of simulations and experiments that the step different below 0.5 μm did not necessarily suppress the disclination problem caused by the step difference depending on the conditions or configurations of the LCD device.